At least 500 people were injured in the attacks.
Claiming responsibility for the bombings via its Amaq news portal, ISIS said the blasts were in retaliation to the recent firing at two mosques in New Zealand which led to the death of 50 Muslims.
Prime Minister Ranil Wickremesinghe said Tuesday that police were hunting for more armed suspects and that further attacks were possible.
The Sri Lankan government had first placed the blame on local religious group National Thowheed Jamaath.
One of the additional bombers was the wife of another bomber, he said.
Earlier Sri Lanka's deputy defence minister Ruwan Wijewardene told reporters that eight out of nine suicide bombers had been identified adding that they were all well educated and belonged to wealthy families.
"A security source told Amaq agency the perpetrators of the attack targeting the citizens of [US-led] coalition countries and Christians in Sri Lanka were Islamic State fighters", it said.
Sri Lanka's president has vowed to conduct a major shake-up of the country's security establishment after its failure to prevent the Easter Sunday bombings that killed more than 320 people, despite some officials apparently having prior information about the attacks.
The US embassy in Colombo said agents from the Federal Bureau of Investigation (FBI) were on the ground to help Sri Lankan authorities with the investigation. The prime minister has said neither he or his cabinet ministers were informed of the terror threat, raising questions on if this attack could have been avoided.
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Junior minister Ruwan Wijewardene also told reporters that numerous bombers were highly educated and came from well-off families.
The president announced on Wednesday that he planned to change the head of the defense forces within 24 hours.
"A certain foreign intelligence service has reported that the leader of the National Tawheed Jamath - NTJ, Mohamed Cassim Mohamed Zahran, also known as Zahran Hashim and his followers - have been planning for suicide attacks within Sri Lanka", the memo reads. He also pledged "a complete restructuring" of the security forces.
But the government faces anger over revelations that specific warnings about an attack went ignored.
The government has said the attacks were carried out by Islamic fundamentalists in apparent retaliation for the New Zealand mosque massacre last month, but has said the seven bombers were all Sri Lankan.
An Australian white supremacist was arrested in the Christchurch shootings.
The group were united in their belief that Islam should be the only religion in Sri Lanka, and that was what motivated their attack on Sunday.
More than 300 men, women and children were killed in the multiple bombings that ravaged Sri Lanka on Easter Sunday.
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The vast majority of those killed in the blasts were Sri Lankan, many from the island nation's Christian minority.